1. Technical Field
The present disclosure relates to a method of manufacturing a semiconductor device.
2. Description of Related Art
In a semiconductor device, for example, two or more members such as a semiconductor element or a lead frame are joined to each other using a solder. In a case where two members are joined to each other using a solder, a technique of providing a Ni film such as a nickel (Ni) plating on a surface of each of the members has been widely performed in order to prevent excessive growth of an intermetallic compound in a joint interface between the two members. However, in a case where the Ni film is exposed to a high temperature of, for example, 200° C. for a long period of time, an intermetallic compound (for example, Ni3Sn4) may be produced between the Ni film and the solder. In this case, the joining strength decreases.
Japanese Patent Application Publication No. 2007-67158 (JP 2007-67158 A) discloses a method of manufacturing a semiconductor device using a Sn—Cu solder containing Cu6Sn5. This manufacturing method includes: a step of disposing a Sn—Cu solder between two members; and a step of producing (Cu,Ni)6Sn5 on a Ni film of each of the members by heating and melting the Sn—Cu solder. According to this manufacturing method, (Cu,Ni)6Sn5 produced on the Ni film functions as a barrier layer which prevents conversion of the Ni film into an intermetallic compound such that a decrease in the joining strength of a joint interface between the solder and the Ni film can be prevented.
In the manufacturing method disclosed in JP 2007-67158 A, by melting the solder between two members, (Cu,Ni)6Sn5 is produced on the respective Ni films of the two members at the same time. In this case, while the solder is melted between the two members, a portion of Cu6Sn5 included in the solder moves to one member to produce (Cu,Ni)6Sn5, and another portion of Cu6Sn5 moves to the other member to produce (Cu,Ni)6Sn5. Here, it is difficult to uniformly heat the solder disposed between the two members. For example, a temperature distribution of the solder during heating may be non-uniform in a thickness direction thereof. At this time, in a region of the solder contacting one member, an increase in the temperature of the solder is rapid, and the solder is melted quickly; whereas, in a region of the solder contacting the other member, an increase in the temperature of the solder is slow, and the solder is melted slowly. In this case, the production of (Cu,Ni)6Sn5 is started in one member, whereas the production of (Cu,Ni)6Sn5 is delayed in the other member. As a result, most of Cu6Sn5 included in the solder is consumed in one member, and thus a predetermined amount of (Cu,Ni)6Sn5 may not be produced in the other member.
In order to address this problem, for example, a technique of increasing the Cu concentration in a Sn—Cu solder may be considered. As the Cu concentration increases, the amount of Cu6Sn5 included in the solder also increases. When the solder contains an abundant amount of Cu6Sn5, the amount of (Cu,Ni)6Sn5 required for each surface of two members can be produced even in a case where (Cu,Ni)6Sn5 is non-uniformly produced between the two members. However, as the Cu concentration increases in a Sn—Cu solder, the melting temperature (liquidus temperature) also increases (refer to FIG. 22). Accordingly, when the Cu concentration of a Sn—Cu solder used increases, it is necessary to heat the solder to a higher temperature in a solder melting step. For example, in the manufacturing method disclosed in JP 2007-67158 A, a Sn—Cu solder containing 3 wt % to 7 wt % of Cu is used, and the melting temperature thereof is about 330° C. to 400° C.